Recently, an absorbent resin is widely used in sanitary goods such as paper diaper, sanitary napkin, incontinence pad, etc., for the purpose of absorbing liquid.
Examples of known absorbent resins include: a partially neutralized crosslinked polymer of polyacrylic acid (Japanese Laid-Open Patent Publication No. 84304/1980 (Tokukaisho 55-84304), Japanese Laid-Open Patent Publication No. 108407/1980 (Tokukaisho 55-108407) and Japanese Laid-Open Patent Publication No. 133413/1980 (Tokukaisho 55-133413)); a hydrolyzed graft polymer of starch-acrylonitrile (Japanese Examined Patent Publication No. 43995/1974 (Tokukosho 46-43995)); a neutralized graft polymer of starch-acrylic acid (Japanese Laid-Open Patent Publication No. 125468/1976 (Tokukaisho 51-125468)); a saponified copolymer of vinyl acetate-acrylic ester (Japanese Laid-Open Patent Publication No. 14689/1977 (Tokukaisho 52-14689)); a hydrolyzed copolymer of acrylonitrile or of acrylamide (Japanese Examined Patent Publication No. 15959/1978 (Tokukosho 53-15959)); a crosslinked polymer of cationic monomer (Japanese Laid-Open Patent Publication No. 154709/1983 (Tokukaisho 58-154709) and Japanese Laid-Open Patent Publication No. 154710/1983 (Tokukaisho 58-154710)).
Notable properties of the absorbent resin include absorbing capacity and absorbency under pressure when it is in contact with an aqueous liquid like a body fluid, gel strength, and absorbing power of absorbing liquid from a base material containing an aqueous liquid, etc. Conventionally, the absorbent resins having some of the above-mentioned properties which show desirable properties (absorption properties) in their applications of paper diaper, sanitary napkin, etc., as well as absorbent materials and the absorbent product using such absorbent resins have been proposed.
Examples of such absorbent resins, absorbent materials and absorbent products include: an absorbent resin disclosing a combination of a specific gel capacity, shear modulus and an extraction polymer content (U.S. Pat. No. 4,654,039); an absorbent resin having predeterined ranges of absorbing capacity and absorbency under pressure and gel stability, and a paper diaper and a sanitary napkin using such absorbent resin (Japanese Laid-Open Patent Publication No. 185550/1985 (Tokukaisho 60-185550)), (Japanese Laid-Open Patent Publication No. 185551/1985 (Tokukaisho 60-185551)) and (Japanese Laid-Open Patent Publication No. 185804/1985 (Tokukaisho 60-185804)); a paper diaper using an absorbent resin having predetermined ranges of absorbing capacity, absorbency under pressure and a gel stability (Japanese Laid-Open Patent Publication No. 185805/1985 (Tokukaisho 60-185805)); an absorbent product having predetermined absorbing capacity, absorbing power and water soluble content (Japanese Laid-Open Patent Publication No. 21902/1988 (Tokukaisho 63-21902)); an absorbent sanitary product including an absorbent resin having predetermined ranges of absorbing capacity, absorbing capacity under pressure, and a gel breaking strength (Japanese Laid-Open Patent Publication No. 99861/1988 (Tokukaisho 63-99861)); a paper diaper including an absorbent resin having predetermined ranges of absorbing capacity and absorbency under pressure (Japanese Laid-Open Patent Publication No. 34167/1990 (Tokukaihei 2-34167)); an absorbing agent containing an absorbent resin having predetermined ranges of absorbing capacity under pressure and particle diameter (European Patent No. 339,461); an absorbing agent including at least a predetermined amount of an absorbent resin having predetermined ranges of absorbency under pressure and absorbing capacity under pressure in a short period of time (European Patent No. 443,627); an absorbing synthethic material having at least predetermined amount of absorbent resin having predetermined ranges of deformation and absorbing index when applying a load (European Patent No. 532,002).
Recently, still thinner sanitary goods such as a paper diaperand a sanitary napkin of higher quality have been demanded, and an amount of use of the absorbent resin or weight percent of the absorbent resin in the absorbent material mainly composed of absorbent resin and hydrophilic fiber (hereinafter simply referred to as a resin concentration) has been increasing. Namely, by reducing the amount of hydrophilic fiber having a small bulk specific gravity and increasing the amount of the absorbent resin having a large bulk specific gravity, the ratio of the absorbent resin in the absorbent material is raised, and a thinner sanitary material can be achieved without reducing the absorbing capacity.
However, earnest researches have been made by the inventors of the present invention in order to increase the absorbing amount of the sanitary material, for example, by increasing the resin concentration in the absorbent material. As a result, the inventors have found that the absorbent material having a higher resin concentration than conventional absorbent material cannot be used without having problems such as leakage of aqueous liquid from the sanitary goods, etc., only by controlling the described absorbing capacity, absorbency under pressure, gel strength, absorbing capacity, etc. For example, as to the absorbent resin which has been viewed with interest in which only absorbing capacity under pressure is large, by raising the resin concentration, the liquid dispersibility of the absorbent material is significantly reduced.
Further researches have been made by the inventors on absorbing properties of the absorbent material having higher resin concentration than conventional absorbent material. As a result, they have found that in the case of using a mixture of a known absorbent resin, a hydrophilic fiber is used as an absorbent material, although when a resin concentration is low, the absorbent material shows a predetermined level of absorbing properties, when a resin concentration is above 40 percent by weight, such unpreferable conditions that the liquid dispersibility is rapidly lowered, the absorbing capacity of weight per unit of the absorbent material is lowered, the water absorbing capacity cannot be ensured for a long period of time; an amount of wet back of the aqueous liquid increases, etc., would occur. Namely, when a mixture of the known absorbent resin and the hydrophilic fiber is used as an absorbent material, the above-mentioned problems would arise.
Accordingly, an object of the present invention is to eliminate the above-mentioned problems and to provide an absorbing agent having excellent properties (absorbing properties) such as very high liquid diffusivity and a stability over time of/the absorbing capacity irrespectively of the resin concentration or the structure of the absorbent material, for example, when it is used as sanitary goods, etc., as well as a process of manufacturing such absorbing agent and the absorbent product including the absorbing agent.
Another object of the present invention is to provide an absorbing agent having excellent properties (absorbing properties, etc.,) of maintaining very high liquid diffusivity and absorbing capacity even with high resin concentration.